1. Field of the Invention
The present invention relates to a DSRC (dedicated short-range communication) car-mounted equipment used for an ETC (electronic toll collection) system in an ITS (intelligent transport system). More particularly, the invention relates to a DSRC car-mounted equipment which prevents communication error relative to an on-the-road equipment by setting an optimum communication area depending upon the vehicle speed data.
2. Prior Art
There has heretofore been known a DSRC car-mounted equipment (hereinafter also referred to simply as xe2x80x9ccar-mounted equipmentxe2x80x9d) for transmitting and receiving a variety of data through communication with an on-the-road equipment installed on a road on which a vehicle travels, and has been used as, for example, an ETC car-mounted equipment.
FIG. 4 is a block diagram schematically illustrating the constitution of a conventional DSRC car-mounted equipment and illustrating, here, an ETC car-mounted equipment.
FIG. 5 is a diagram of characteristics showing electric field intensities received by the conventional DSRC car-mounted equipment, and FIG. 6 is a flowchart illustrating the communication operation between the on-the-road equipment and the car-mounted equipment using the conventional DSRC car-mounted equipment.
In FIG. 4, the car-mounted equipment 1 includes a microcomputer 10 for processing data received from the on-the-road equipment, a communication control unit 11 that operates in relation to the microcomputer 10, a radio detector unit 12 controlled by the communication control unit 11, and a storage medium interface 13 for sending and receiving a variety of data (such as toll collection data) to and from an external storage medium 2.
The external storage medium 2 may be an IC card having a car-mounted interface 20, and sends and receives data related to the toll collection to and from the microcomputer 10 in the car-mounted unit 1 through the storage medium interface 13.
Here, through not diagramed, the storage medium interface 13 interposed relative to the external storage medium 2 includes a physically mounted unit and a control unit (not shown) for reading data, and is allowed to be connected to various external equipment.
The conventional car-mounted equipment is constituted as shown in FIG. 4 and in which the microcomputer 10 controls the communication control unit 11 to transmit and receive real radio data to and from the on-the-road equipment (not shown) through the radio detector unit 12.
That is, the microcomputer 10 fetches the data received from the on-the-road equipment through the radio detector unit 12, and sends transmission data to the on-the-road equipment through a transmission unit (not shown).
The radio detector unit 12 in the car-mounted equipment 1 detects the radio electric field intensity transmitted from the on-the-road equipment, and informs the microcomputer 10 of the fact that the car is entering the area for communicating radio data for toll collection.
In the automatic toll collection system as is widely known, the state in which the external storage medium 2 is normally mounted on the car-mounted equipment 1 and the toll collection conformance of the external storage medium 2 are confirmed and, then, the radio communication is executed relative to the on-the-road equipment using the data within the communication area, in order to automatically collect the toll.
In the DSRC car-mounted equipment 1 of this kind, in general, the sensitivity for reception has been set constant irrespective of whether it is within the communication area or not. Therefore, the electric field intensity of the electromagnetic waves received from the on-the-road equipment varies depending upon the distance (position in a direction in which the vehicle is traveling) as shown in FIG. 5.
In FIG. 5, the abscissa represents the position (distance in the direction in which the vehicle is traveling) of the car-mounted equipment with respect to an antenna ANT of the on-the-road equipment, and the ordinate represents the intensity of electric field received by the car-mounted equipment 1. The communication start area (communication area) is set relying upon a relationship between the electric field intensity determined by the distance from the antenna ANT of the on-the-road equipment and a predetermined level (threshold) TH corresponding to the sensitivity for reception.
Here, the antenna ANT of the on-the-road equipment is installed at a toll collection gate of a toll expressway.
The communication area in which the data are exchanged after the vehicle has entered into the communication start area is set within about 4 meters from the antenna ANT of the on-the-road equipment.
Further, the communication area set by the predetermined level TH (sensitivity for reception) may include areas A, B where the electric field intensity so drops that the communication cannot be accomplished and unstable boundary areas due to side lobes in the output from the antenna ANT of the on-the-road equipment.
As the vehicle approaches the on-the-road equipment, the communication is repeated a plural number of times between the on-the-road equipment and the car-mounted equipment 1, and the on-the-road equipment repetitively transmit communication signals to the car-mounted equipment 1 at all times.
Next, the communication operation of the conventional DSRC car-mounted equipment will be described with reference to FIGS. 4, 5 and 6.
FIG. 6 illustrates the flow of the toll collection communication by taking into consideration the areas A, B where the electric field intensity drops.
In FIG. 6, first, the car-mounted equipment 1 approaches the on-the-road equipment 1 and enters into the system start area and where the electric field intensity received from the antenna ANT of the on-the-road equipment becomes greater than a system drive level ( less than predetermined level TH). Then, the system in the car-mounted equipment 1 is driven in response thereto (step S11).
Then, the car-mounted equipment 1 further approaches the antenna ANT of the on-the-road equipment, whereby the intensity of the received electric field exceeds the predetermined level TH and a first communication signal is received. Then, in response thereto, the car-mounted equipment 1 transmits communication signals to the antenna ANT of the on-the-road equipment and executes the initial communication for receiving the toll (step S12).
Then, it is judged whether the communication for receiving the toll is all completed between the on-the-road equipment and the car-mounted equipment 1 (step S13). When the communication signals are normally exchanged and it is judged that the communication has completed (i.e., YES), the routine normally ends in a state where the communication for collecting the toll has been finished (step S14).
When it is judged at step S13 that the communication for receiving the toll has not all been completed (i.e., NO), the on-the-road equipment repetitively transmits retrial communication signals for collecting the toll to the car-mounted equipment 1 (step S15).
Next, it is judged whether the communication is normal between the on-the-road equipment and the car-mounted equipment 1 (step S16). When it is judged that the communication is normal during the above retrial period (i.e., YES), the routine returns back to the step S13 and the processing is repetitively executed until it is confirmed that the toll collection communication has normally ended (step S14).
Here, the retrial communication operation by the on-the-road equipment is repeated about 100 to 200 times for every 2 milliseconds, and it is judged that the communication condition is normal when the vehicle enters into the communication-possible area (or when the vehicle leaves the boundary area).
At step S16, on the other hand, when the vehicle enters into the area A or B (or boundary area) where the electric field intensity drops and it is judged that the communication condition is not normal (i.e., NO), it is then judged whether the retrial communication period has elapsed (step S17).
When it is judged that the retrial period has not been elapsed (i.e., NO), the routine returns back to the step S13 to repeat the above-mentioned processings. When it is judged that the retrial period has elapsed (i.e., YES), it is so regarded that the communication for toll collection is impossible, and the routine abnormally ends (step S18).
That is, when there is quite no response from the car-mounted equipment 1 after it is confirmed that the car-mounted equipment 1 has entered into the communication area, the on-the-road equipment regards that the communication is impossible (car-mounted equipment 1 is not existing) and ends the communication.
Thus, as the electric field intensity detected by the radio detector unit 12 exceeds a predetermined level TH, the microcomputer 10 in the car-mounted equipment 1 informs that the car-mounted equipment 1 is within the communication area for collecting the toll and commences the communication operation.
As described above, however, the radio communication may be successful or unsuccessful in the areas A, B where the electric field intensity drops and in the boundary area where the electric field level is near the predetermined level TH for setting the received electric field intensity due to dispersion in the level of the electric field.
Further, the communication-impossible regions due to the areas A, B where the electric field intensity drops are greatly related to the vehicle speed.
That is, due to traffic jam or some trouble, the vehicle mounting the equipment 1 that has entered into the communication area (not smaller than the predetermined level TH) may stay long (or may move at a very low speed) in the areas A, B where the electric field intensity drops during the retrial communication (which is a period lasting for about 0.5 seconds) effected plural times.
Further, the vehicle running at a low speed due to the concentration of traffic may stay longer in the areas A, B where the electric field intensity drops, resulting in a succession of radio communication failure, causing the on-the-road equipment to so judge that the car-mounted equipment is abnormal making it difficult to automatically collect the toll.
In this case, when the predetermined level TH remains constant as shown in FIG. 5, the communication signals 3 are not received from the on-the-road equipment, and the state where there is no response from the car-mounted equipment 1 continues, spoiling the function of the car-mounted equipment.
Further, when the communication area is contracted by setting the predetermined level TH high in order to prevent the occurrence of inconvenience during the low-speed running, the vehicle that is running at a high speed stays short in the communication area, which may make it difficult to complete the communication within the communication area for collecting the toll.
According to the conventional DSRC car-mounted equipment as described above, the communication area is set in a fixed manner relative to the on-the-road equipment. Therefore, if the communication area is widely set, communication becomes impossible when the vehicle is traveling at a low speed being affected by the areas A, B where the electric field intensity drops. If the communication area is narrowly set, on the other hand, it becomes difficult to complete the communication within the communication area when the vehicle travels at a high speed. In either case, the car-mounted equipment cannot be effectively utilized.
The present invention was accomplished in order to solve the above-mentioned problems and has the object of providing a DSRC car-mounted equipment capable of reliably preventing communication error by variably setting the communication area depending upon the vehicle speed.
The DSRC car-mounted equipment according to the present invention comprises:
a communication control unit for transmitting and receiving data by radio to and from an on-the-road equipment installed on a road on which a vehicle travels;
a radio detector unit for detecting signals received from the on-the-road equipment by detecting the intensity of the radio electric field; and
a microcomputer for fetching output signals from the radio detector unit by controlling the communication control unit to process data included in the received signals; wherein
the microcomputer includes:
an input interface for receiving data related to the speed of the vehicle, and
communication area-setting means for variably setting the radio communication area depending upon the vehicle speed data; and wherein
the communication area-setting means contracts the communication area when the vehicle speed data represents a low-speed state and expands the communication area when the vehicle speed data represents a high-speed state.
In the DSRC car-mounted equipment according to the invention, the communication area-setting means includes comparator means for comparing the intensity of the radio electric field with a predetermined judging level to determine the communication area, and increases the judging level when the vehicle speed data represents a low-speed state and decreases the judging level when the vehicle speed data represents a high-speed state.
In the DSRC car-mounted equipment according to the invention, the communication area-setting means changes the judging level in two steps by setting the judging level to a first judging level for low speed when the vehicle speed data represents a low-speed state, and setting the judging level to a second judging level for high speed when the vehicle speed data represents a high-speed state.
In the DSRC car-mounted equipment according to the invention, the communication area-setting means compares the vehicle speed data with a predetermined value corresponding to any speed within a range of from 5 km to 15 km per hour, contracts the communication area to become smaller than a normal communication area when the vehicle speed data is smaller than the predetermined value, and expands the communication area to become larger than the normal communication area when the vehicle speed data is not smaller than the predetermined value.
In the DSRC car-mounted equipment according to the invention, the communication area-setting means compares the vehicle speed data with a first predetermined value and with a second predetermined value larger than the first predetermined value, contracts the communication area to become smaller than a normal communication area when the vehicle speed data is smaller than the first predetermined value, and expands the communication area to become larger than the normal communication area when the vehicle speed data is larger than the second predetermined value.
The DSRC car-mounted equipment according to the invention further comprises an external storage medium connected to the microcomputer to exchange data related to the collection of the toll, wherein the microcomputer exchanges data related to the collection of the toll between the on-the-road equipment installed on a toll road and the external storage medium, and automatically executes the toll collection processing based on the data related to the collection of the toll.